FIG. 7 is a schematic view of a conventional power supply circuit. The power supply circuit 1 includes a first input terminal 2, a second input terminal 3, a relay 4, a main power supply 5, an assistant power supply 6, a microprocessor 7, and a switch 8. The assistant power supply 6 includes a transformer 9 and a commutating and filter circuit (not labeled).
The first input terminal 2 is connected to the main power supply 5, and the second input terminal 3 is connected to the main power supply 5 via the relay 4. The main power supply 5 is connected to a load (not shown). The transformer 9 includes a primary winding (not labeled) and a secondary winding (not labeled). The primary winding includes two terminals (not labeled). One of the terminals of the primary winding is connected to the first input terminal 2, and the other terminal is connected to the second input terminal 3. The secondary winding is connected to the microprocessor 7 via the commutating and filter circuit. The relay 4 includes an inductance coil (not labeled), and the inductance coil includes two terminals (not labeled). One of the terminals of the inductance coil is connected to the microprocessor 7, and the other terminal is connected to ground. The microprocessor 7 is connected to ground via the switch 8.
An alternating current (AC) voltage is inputted into the assistant power supply 6 and converted into a direct current (DC) voltage by the transformer 9 and the commutating and filter circuit. The DC voltage is supplied to the microprocessor 7 to enable the microprocessor 7 to function.
When a user presses the switch 8, the switch 8 correspondingly generates a first pulse signal. The microprocessor 7 receives the first pulse signal and correspondingly outputs a first control signal to turn on the relay 4. The AC voltage is inputted into the main power supply 5 via the first input terminal 2, the second input terminal 3, and the relay 4. The main power supply 5 converts the AC voltage into required voltages to supply the load.
When the user presses the switch 8 again, the switch 8 correspondingly generates a second pulse signal. The microprocessor 7 receives the second pulse signal and correspondingly outputs a second control signal to turn off the relay 4. The main power supply 5 outputs no voltage, and the load stops operating correspondingly. That is, the power supply circuit 2 is in a stand-by state.
Although the power supply circuit 2 is in the stand-by state, the AC voltage is still inputted into the assistant power supply 6. The DC voltage outputted from the assistant power supply 6 is still supplied to the microprocessor 7. That is, when the power supply circuit 2 is in the stand-by state, energy consumption is large.
What is needed, therefore, is a power supply circuit that can overcome the above-described deficiencies.